In recent years, there has been an increasing concern with the quality of the air, especially with health-related aspects such as "Sick Building Syndrome", and the concern for odors within buildings and other structures. These concerns have become more acute with the advent of energy-related trends for reduction of air exchange rates within buildings which increase stale, odorous air, and potentially harmful components in building air. This, in turn, has led to an increased interest in systems and devices to reduce the amount of these undesirable contaminants in breathing air.
The undesirable materials sought to be removed from air are generally found in two fundamental forms: particulate and gas or vapor. For particulate removal, a number of processes are available and currently practiced, including barrier filtration, electrostatic precipitation, etc.
For the gas and vapor components, which are frequently organic compounds, technologies involving activated carbon adsorption are typically recommended. Physical adsorption on activated carbon is the most efficient means of removing a mixture of a wide variety of contaminants from air at levels in the part per million by volume (ppmv) or lower concentrations. There are several means of applying activated carbon, each with its associated advantages and disadvantages.
Frequently, granular activated carbon (GAC) or pelletized activated carbon is placed in trays, either loose, or held in place by a retention screen, and placed within the air stream of a building HVAC system. Alternately, the carbon can be placed within an air-handling device sized to treat the air in a single room. The particle size of the carbon is relatively large, several millimeters in diameter, to increase the size of the void spaces between the particles and thus reduce the pressure drop at a given linear velocity of air. However, the large particle size also increases the length of the diffusion path a contaminant molecule must travel, and therefore the time to adsorb. Consequently, the residence time of the contaminated air in contact with the GAC must be increased proportionately. Problems with such systems include high pressure drops, and the need for periodic replacement of the carbon as its capacity is spent. Such replacement can be laborous and potentially dangerous if harmful or hazardous materials are removed by the systems.
An alternative to GAC is powdered activated carbon (PAC). PAC has a 50 to 100 times smaller particle size, and thus shorter diffusion paths for adsorption. As a result, the residence time the gas must be in contact with the carbon bed is reduced proportionately. This allows for very thin bed depths of millimeter thicknesses. However, PAC is very difficult to contain, and the pressure drop across the bed can be extremely high.
Some of these handling issues have been addressed by enclosing the otherwise loose carbon (GAC or PAC) within a matrix of some sort. Thus, the carbon can be bonded to itself or to a support structure to form a self-supporting block, panel or slab (WO 94/03270, PCT/US93/06274). It can also be adhered to fibers in a woven or nonwoven web structure. The carbon can then be handled as a number of carbon media units, rather than as a loose material. Pressure drops by the media are addressed by providing void spaces within the matrix. The spacing of the carbon particles decreases the pressure drops to acceptable levels, but the efficiency of the media filter is reduced because a substantial portion of the air passes through the filter without contacting a carbon particle. This solution, however, does not address the issue of a finite adsorption capacity of the carbon for the contaminants. Consequently, the need for replacement remains. In fact, the process of binding the carbon frequently causes a reduction in capacity, as some of the carbon surface is occluded by the adhesive.
Methods are also known whereby the capacity of activated carbon, or an activated carbon media can be substantially regenerated. This reduces the frequency with which maintenance is required. Alternatively, regeneration permits the use of smaller quantities of carbon which reduces capital cost and space requirements without an associated reduction in effectiveness. The process of regeneration is frequently accomplished by heating the carbon bed by some means. It is known in the art that the capacity of an activated carbon for materials removed by the mechanism of physical adsorption is decreased at elevated temperature. Thus, when the temperature of a quantity of activated carbon which has largely been loaded to its saturation with a given contaminant is increased, the contaminant will desorb from the pore structure of the carbon, and can be swept away with a suitable purge stream. Thus, when the carbon is cooled, a significant portion of the original capacity is restored. The actual temperature of the internal carbon structure at any point in time dictates the adsorption capacity of the contaminants and thus the amount of desorption, and the capacity recovered for the next adsorption cycle. Commonly, the heat necessary to warm the carbon is supplied externally. Thus the temperature of the external heat source must always be greater than or equal to the activated carbon structure. The carbon bed is commonly heated by application of hot air, such as by heating a sweep gas, or with steam. It can also be heated by placing heating elements in contact with the carbon particles or carbon media JP 51 135896.
It is known in the art that activated carbon, because of its localized graphite-like structure, is capable of conducting electricity. It is also known that the resistance properties are such that useful heat can be generated in this manner. Thus, some attempts have been reported to utilize this property to generate the heat necessary to achieve regeneration of activated carbon beds (DE 4104513). Unfortunately, this method has generally been attempted with beds of granular or pelleted carbon, or media derived therefrom and they have met with only limited success. Typically encountered problems include non-uniform heating patterns, hot-spots, and short-circuits.
In addition to the well-known traditional physical forms of activated carbon (i.e., granular, pelletized, spherical, powdered), it is also known that activated carbon can be prepared in the form of activated carbon cloth (ACC) or activated carbon felt (ACF). This adsorption media consists of activated carbon in the form of woven or knitted (ACC), or loose mat (ACF) activated carbon fibers. The fibers have a diameter similar to PAC, and therefore provide diffusion paths and adsorption rates similar to PAC. The advantage of ACF and ACC is that they are easy to apply in very thin beds of millimeter dimensions, like the PAC bonded to supports, with adequately low pressure drops, but with efficiencies as high as the deeper GAC beds. Because the fibers of the ACC can be of very small diameter, and because the pressure drop across a number of layers of cloth can be small, the ACC has dynamic properties which are well suited to the problem of air purification. The ACC and ACF forms, however, suffer from the same limitations as to their adsorption capacity of the other forms of activated carbon. Thus, the time between replacements can be unacceptably short. Some have attempted to regenerate ACC and ACF media by heating with air, or by placing the media in contact with an electrical heater (JP 2046852, JP 2046848).
Accordingly, it is an object of the present invention to provide a means and method for enhancing the purity of air stream without the attendant disadvantages inherent in the prior art methods. It is a further object of the invention to provide a method to remove contaminates in air streams using a woven and knitted ACC that can be regenerated very effectively and uniformly by directly heating with an electrical current. It is also an object of the invention to provide a method and apparatus for continuously adsorbing organic materials and other contaminates from an air stream and subsequently regenerating the capacity of the ACC.